Cholesteric liquid crystals (ChLCs) exhibit a director that is helically twisted around an axis perpendicular to the long axis of the molecules. Due to this supramolecular arrangement and the inherent birefringence of the LC molecules, ChLCs have very interesting optical properties. For example, they reflect 50% of incident non polarized light as circularly polarized light of a specific wavelength and with the same handedness as the helix, while the other 50% of the incident light will be transmitted through the cholesteric helix. The central wavelength of the reflected light λ is related to the average refractive index n of the ChLC material and the pitch p of the helix by the following equation:λ=p·n 
As the helical pitch p is dependent on the temperature, the wavelength λ of the reflected light will also vary with varying temperature. This temperature dependence of the reflection wavelength can be used for example in thermochromic applications. However, at the same time this can also be an inconvenience for the stability of the optical performance for specific applications that need a stable optical effect, such as cholesteric films or cholesteric pigment flakes for use in colour filters, reflective polarisers, security elements, cosmetic products or paints.
For these applications it has therefore been suggested in prior art to use polymerized ChLCs, which are polymerized or crosslinked in their cholesteric phase. As a result the helically twisted molecular structure is permanently fixed, and the reflection wavelength becomes temperature independent.
For preparing cholesteric polymer foils it is necessary to provide the polymerizable ChLC material on a substrate, align the ChLC molecules into macroscopically uniform orientation, and then polymerize the aligned ChLC material. However, this method is time- and cost-consuming, and especially high effort is often required to ensure good uniform alignment of the ChLC within the polymerized foil.
It has also been suggested in prior art to use flat cholesteric pigment flakes, which can be used as pigments for optical, decorative or security applications, for example in cholesteric foils or printing inks. Such cholesteric flakes are usually prepared from a polymerized ChLC foil as mentioned above, which is removed from the substrate and be crushed or milled into flakes of the desired size. Again this method is time- and cost-consuming and, besides the effort needed to create unfirm alignment within the flakes, it is also necessary to apply specific milling and/or sieving techniques to obtain flakes of uniform size and shape. Also, the ChLC material in the flat flakes is oriented usually with the cholesteric helix axis perpendicular to their thickness direction. Therefore, when forming a sheet or foil from a coating comprising the flakes it has to be ensured that the flakes are coated such that they are oriented mainly parallel to the substrate to ensure good quality of reflection.
For example, prior art discloses cholesteric pigment flakes obtained by crushing polymerized films in WO 1997/000600 A2, DE 19602848 A1, WO 2008/128714 A1 and JP 2005-187542 A1, cholesteric pigment particles obtaining by template techniques in DE 19602795 A1, encapsulated ChLCs that shift the colour with the temperature in CA 1108838, the use of such ChLCs for decorative applications or in cosmetics in CH 491533 and US 2009/0190091 A1, cholesteric droplets dispersed in a continuous polymeric matrix for enhancing the contrast colour in LCDs in U.S. Pat. No. 3,734,597, or non liquid crystalline particles (organic or inorganic) coated by a cholesteric layer in WO 2012/666841 A1, JP 11315146 A1, WO 2011/048989 A1, JP 2002155241 A. However, some of the above mentioned products are based on soft-materials such as droplets and encapsulated ChLCs, which makes the pitch sensitive to the temperature and external stimuli, whereas the cholesteric flakes often have an undefined or unregular lateral shape and broad distribution of their lateral dimensions, and therefore require special milling and/or sieving techniques to achieve homogeneous particle size and shape.
Therefore there is still a need for improved ChLC polymer particles which are stable against mechanical, chemical and thermal influence, have temperature independent optical properties, exhibit uniform size and shape, and are easy to prepare. There is also a need for simple, time- and cost-effective methods of preparing layers or articles from such ChLC polymer particles. It is an aim of the present invention to provide such improved layers, articles and ChLC polymer particles, and for improved methods of preparing such layers, articles and ChLC polymer particles.
The inventors of the present invention have found that these aims can be achieved by providing ChLC layers or articles, ChLC polymer particles and methods for their preparation, as disclosed and claimed hereinafter.
In particular, the inventors of the present invention have found that these aims can be achieved by synthesizing solid cholesteric polymer particles by emulsion or suspension photopolymerization of achiral reactive mesogens (RMs) mixed with chiral dopants, or of chiral RMs. These solid cholesteric particles are obtained directly from the polymerization process with the cholesteric order frozen in, so that the optical properties of the cholesteric liquid crystal structure are permanently fixed and remain stable against external stimuli like temperature variation, mechanical stress or chemical agents. Besides, only a very simple filtration and washing process is required for their further use. The particles can be stored as a dispersion in an isotropic or anisotropic continuous phase or as a powder able to be redispersed again when a solvent is added. The continuous phase can be polar or apolar solvent, a polymerizable or polymerized matrix or an anisotropic liquid such as liquid crystals. The liquid or matrix with the dispersed particles can be coated or cast onto a substrate to form a film, foil, or another shaped article, which can be used for example as optical active film, lens, thermally insulating sheet, or as component in an electrooptical device. The particles and/or the liquid or matrix containing these particles can also be directly used in an ink, paint, as pigments or additives, for decorative, cosmetic or security applications or in optical or electrooptical devices or components thereof.
The synthesis of solid cholesteric particles has been very recently reported by Ciparrone et al (Adv. Mater. 2011, 23, 5773-5778, Lab Chip, 2013, 13, 459-467). However, there is no disclosure of a layer or article comprising such particles.